A white-light scanning interferometer is an optical system as shown in FIG. 10. In FIG. 10, a white-light source 1101 is a light source such as a halogen lamp, which emits light of wide-band wavelength distribution. The white light emitted from the white-light source 1101 enters a half mirror 1102. The half mirror 1102 divides the light, and guides the divided light beams to a sample 1103 and to a reference plane 1104, respectively. These light beams are incident upon the sample 1103 and the reference plane 1104 and reflected off therefrom, respectively, and then again overlaid on each other at the half mirror 1102. The overlaid light becomes incident upon an area sensor 1105. At this time, the area sensor 1105 captures images while scanning the reference plane 1104 in a direction of an arrow 1110 in the figure. The light being incident upon is converted into an image, and captured by an arithmetic unit 1106.
Next, with reference to a flowchart shown in FIG. 11, a description will be given of a procedure of measuring the film-thickness distribution using such an apparatus. It is to be noted that, the sample 1103 includes therein a first interface and a second interface.
In FIG. 11, first, in step S201, an image is captured while scanning the reference plane 1104 by the optical system shown in FIG. 10, to extract a luminance change in each pixel of the image. Thus, an interference waveform at each pixel is detected.
Subsequently, in step S202, a peak position of each interference waveform with reflected light at the first interface is calculated. Here, as a method for calculating the peak position, what is used is a method including: calculating an envelope by a low-pass filter, for example; and detecting a scanning position that assumes the maximum value thereof.
Subsequently, in step S203, a peak position of each interference waveform with reflected light at the second interface is calculated.
Subsequently, in step S204, a distance of the reference plane 1104 at the two peak positions calculated in steps S202 and S203 is calculated, and the distance is divided by the refractive index. Thus, the film thickness is calculated.
Finally, in step S205, the calculation result of the film thickness is output, and the measurement ends.
In the foregoing manner, white light is emitted to the sample 1103 and the light reflected off is overlaid on the light reflected off the reference plane 1104, to form an image on the area sensor 1105. As a result, interference fringes appear at a portion on the area sensor 1105 where a distance Z from the half mirror 1102 to the reference plane 1104 and a distance h from the half mirror 1102 to the sample 1103 are equal to each other. With the optical system, when the reference plane 1104 is scanned in the arrow 1110 direction, an interference waveform appears at each measurement point. By detecting the peak of the interference waveform at each measurement point and coupling together, an interference waveform over the entire surface sample 1103 can be obtained. Then, based on the interference waveform, a surface shape distribution of the object is measured. Further, with use of this technique, film thickness measurement of a transparent body can be carried out.
In a case where a transparent film is selected as the sample 1103, as shown in FIG. 12A, reflected light 1112 at a front surface 1111 (the first interface) and reflected light 1114 at a back surface 1113 (the second interface) exist. When the optical system is scanned in a depth direction of the sample 1103, these light beams form separate interference fringes, and a measurement result shown in FIG. 12B is obtained. Since the peak interval of the measurement result corresponds to a thickness t of the sample 1103, the thickness can be measured in a range in which superimposition of interference fringes does not occur.
In this measurement scheme, it is preferable to use the white-light source 1101 with which a coherence length being a range in which interference fringes appear becomes the shortest. Specifically, it is preferable that the intensity spectrum of the white-light source 1101 has the widest possible band, and attains the normal distribution. Accordingly, as the white-light source 1101, a wide-band light source such as a halogen lamp is used, and a filter having a wavelength dependency on the transmittance of light is inserted immediately behind the white-light source 1101 (for example, see Patent Document 1).
Here, in a case where the sample 1115 is structured with a plurality of transparent films 1116 and 1117, and a halogen lamp is employed as the white-light source 1101, the coherence length becomes equal to or more than 1 μm. Therefore, when the film thickness of the sample 1103 is equal to or less than 1 μm, as shown in FIGS. 13A and 13B, superimposition of the interference waveform occurs between the interface 1118 between the transparent film layer 1116 and the transparent film layer 1117 and the interface 1119 between the transparent film layer 1117 and the lower layer thereof.